Rectifiers of different types of construction may be used for supplying direct-current systems from three-phase systems. The present application relates to active or controlled bridge rectifiers, which have active switch elements, for example in the form of known metal-oxide-semiconductor-field-effect transistors (MOSFETs). In motor vehicle electrical systems, bridge rectifiers of a six-pulse design are frequently used in accordance with the three-phase generators normally installed in motor vehicles. The present invention, however, is equally suitable for bridge rectifiers for other phase numbers, e.g., for five-phase generators, and in other usage scenarios.
As explained for example in German Patent Application DE 10 2009 04 6 955 A1, the use of active bridge rectifiers in motor vehicles is desirable, inter alia, because these, in contrast to passive or non-controlled bridge rectifiers, have lower power losses.
A critical case of a fault, however, particularly in active bridge rectifiers, is the load dump. This occurs when, in the case of a highly excited generator and a correspondingly high output current, the load on the generator suddenly drops (e.g., when consumers are switched off) and if this is not absorbed by capacitively acting elements in the direct-current network (e.g. the battery in the motor vehicle electrical system).
Here, in the extreme case, it is possible that the generator continues to supply energy into the network up to a duration of approx. 300 to 500 ms. This must be able to be converted (extinguished) in the bridge rectifier in order to protect electrical components connected in outgoing circuit against overvoltage damage. In passive or non-controlled bridge rectifiers, this protection is normally provided by the rectifier diodes themselves because there the energy loss is able to be converted into heat. In currently available active switch elements, however, for example MOSFETs, it is not possible to emulate these properties completely. Additional protection strategies are therefore required.
In the event of a load dump, for example, some or all phases may be short-circuited by switching the associated switch elements to ground (that is, low side). The ensuing voltage drop, however, may have the result that the energy supply for controlling the switch elements can no longer be provided. In particular, it is no longer possible to reload bootstrap capacitors used for this purpose, particularly in the low side path.
It is therefore desirable to indicate a possibility so that an active rectifier having an overvoltage protection still functions even after switching a short circuit.